I n t e r na l A nato my

Skeletal SystemThe skeletal system (Figure 1) is highly modified to meet the struc-

tural and energetic demands of flight. Throughout the skeleton are

examples of three types of adaptive modifications: rigidity, reduc-

tion and redistribution of mass, and modifications of the limbs for

flight. Rigidity is increased by modifications of the structure of

bones and by fusions between them. To improve balance during

flight, most of the muscle mass controlling the wings and legs are

located near the center of the body, producing a compact form.

Locate the following structures on the mounted pigeon skeletons

and note the modifications described below.

The bonesWhile maintaining strength, most of the bones are pneumatic,

being hollow and filled with air spaces connected to the respiratory

system. Internal struts add strength to some bones such as the

humerus.

SkullThe bones of the skull (Figure 2) are generally fused providing pro-

tection to the brain while being of light weight. A light, toothless

beak replaces the bony, heavy toothed jaw of reptiles. Beaks, of

course, can be highly modified for different types of food and feed-

ing behavior. One important vestige of birds’ ancestral relationship

to reptiles is the jaw joint articulation between the quadrate and

articular bones (compare this to the squamosal-dentary joint in

mammals). Note the large orbits, as sight is an important sensory

mechanism for birds. Within the orbit is the sclerotic ring (this

may be absent in your specimen), which helps to support the eye-

ball. In the posterior part of the skull is a single occipital condyle,

which articulates with the first cervical vertebrae. Reptiles likewise

have a single occipital condyle, while mammals have four.

Neck The necks of birds are very important for body maintenance and

eyesight. Modification for flight has rendered avian forelimbs

almost useless for any task other than flight. To make up for this

lack of forelimb dexterity, the beak is used for many tasks such as

preening feathers. To access hard-to-reach feathers on the back and

tail birds require a flexible neck. Furthermore, as birds have immo-

bile eyes, head movement and flexibility is required to focus on

objects at various distances. Heterocoelous (saddle shaped) verte-

brae in the cervical region facilitate the extreme neck mobility

required to carry out these tasks.

Thorax and SternumPosterior to the cervical (neck) vertebrae are five fused thoracic

vertebrae (Figure 1). Supported by these vertebrae are seven sets of

Internal Anatomy

1

Figure 2. The bones of the avian skull (a) lateral view (b) posterior

view (c) sclerotic ring.Carpometacarpus

Ulna

Radius

Scapula

Humerus

Synsacrum

Pygostyle Triosseal canal

Coracoid

Furcula

Uncinate Process

Sternum

Tibiotarsus

Tarsometatarsus

Thoracic vertebrae

Sternal rib

Vertebral rib

Carina (keel)

Radial

UlnarePollex

Ilium

Ischium

Pubis

Femur

Figure 1. The bones of the avian skeleton.

a

b cSclerotic ring

Parietal region

Squamosal region

Occipital crest

Postorbital process

Frontal

Dentary

Nasal aperature

Articular

QuadrateQuadratojugal

Zygomatic arch

Occipital

Pterygoid

Interorbital septum LacrimalNasal

Mesethmoid

MaxillaNasal processof the premaxilla

Maxillary processof the premaxilla

Frontal

Squamosalregion

Periotic capsule

Parietal region

Occipital crest

Occipital complex

Occipital condyle Foramen magnum

two-pieced ribs, composed of a vertebral rib and a sternal rib.

Overlying flaps projecting off the ribs called uncinate processes

help to stiffen the rib cage.

The sternum is the highly modified breastbone. In flying and

swimming birds the keel or carina is enlarged for flight muscle

attachment (called a carinate sternum). Flightless birds in the

order Struthioniformes such as Ostriches have a keelless or “ratite”

sternum.

Pectoral GirdleThe pectoral girdle (Figure 3) is made up of the sternum, clavicle,

coracoid and scapula. The clavicles come together to form the fur-

cula, or “wishbone”. The furcula provides a flexible attachment site

for the breast muscles and along with the coracoids act as struts

that resist pressure created by the wing stroke during flight. Flight

muscles running from the sternum to the relatively short and stiff

humerus elevate and depress the wing. The tendon of the wing-lift-

ing supracoracoideus muscle runs through the trioseal canal found

on the dorsal end of the coracoid, where the coracoid, scapula and

clavicles meet.

Pelvic Girdle Again, there is an extensive fusion of bones of the pelvic region to

provide stiff support for the legs in order to deal with the stress of

take-off and landing. The three pelvic bones, the ilium, ischium

and pubis are fused to form the innominate bone (Figure 4). Fused

dorsally to the innominate bone is the synsacrum to make one

complete unit of bone. The synsacrum is a fusion of the sacral

(pelvic) and 6 caudal (tail) vertebrae. At the end of the spinal col-

umn is the pygostyle, a fusion of the final few caudal vertebrae. The

pygostyle supports the tail feathers and musculature.

WingThe avian wing contains the usual arm bones of reptiles and mam-

mals, but in a highly modified form (Figure 5). The humerus is

rather short compared to the total length of the wing, as it must

withstand the pulling of the flight muscles. Note the large crests on

the proximal humerus for the attachment of the flight muscles.

The radius (slightly thinner than the ulna) and ulna form the sup-

port for the mid-wing. On the ulna are small bumps, the attach-

ment sites for the secondary feathers. Two separate wrist bones, the

radiale and ulnare, are immediately distal to the radius and ulna.

The distal (outer) wing or “hand” bones are highly fused for

strength and feather support and form the carpometacarpus. The

first digit or pollex supports the alula, a small feather used to con-

trol air flow around the wing.

Leg and Foot The upper leg is composed of a fairly standard femur, but the lower

leg and foot are highly modified by fusion (Figure 6). Distal to the

femur is an extremely reduced fibula. Next to this is the tibiotarsus,

a fusion of the tibia (a lower leg bone) and some tarsals (foot

bones). Of course, between the femur and the fibula and tibiotar-

sus is the knee, whose location in birds is often confused. Distal to

the tibiotarsus and fibula is the tarsometatarsus, an extended

fusion of the foot bones. This lengthening adds extra leverage for

running, landing and take-off. Note that birds walk on their toes

(as opposed to foot and toes, as seen in humans) and are thus dig-

itgrade.

MusculatureThe dominant muscles are of course the flight muscles, which can

account for 40% of the body weight of a bird. The mass of muscles,

as in the skeleton, is concentrated ventrally, in the center of gravity,

for aerodynamic stability. Little back musculature is required for

support, as the thoracic, sacral and lumber spinal regions are fused

and consequently very stiff.

The two primary flight muscles are the pectoralis major (largest

muscle in the birds body) and the supracoracoideus (Figure 7).

Thorax and Sternum

e e e 3 3 52

Figure 3. The bones of the pectoral girdle.

Trioseal canal

Scapula

Glenoid fossa

Costal facetsSternal notch

Carina

Clavicle(paired, called

the furcula)

Coracoid

Figure 4. The bones of the pelvic girdle.

Ilium

Synsacrum area(7 sacral vertebrae6 caudal vertebrae)

IschiumPubis

Pygostyle

6 caudal vertebrae

Figure 5. Bones of the arm (dorsal view).

Ulna

Carpometacarpus

First digit, phlanges 1 & 2

Second digit, phalanx 1

Third digit, phalanx 1

Second digit, phalanx 2

Radiale

Ulnare

RadiusHumerus

Trioseal canal

Coracoid

Blade of thescapula

Glenoid fossa

Acromion processof the scapula

Olecranon process of the ulna

The pectoralis major acts to depress the wing in the downstroke. It

originates on the carina of the sternum, furcula and sternal ribs

and inserts (where the muscle acts) on the proximal ventral surface

of the humerus. The supracoracoideus is the primary elevator of

the wing in the upstroke and is most important for take-off. The

supracoracoideus lies internal to or beneath the pectoralis major. It

also originates on the carina of the sternum, the furcula and sternal

ribs. Insertion on the proximal dorsal surface of the humerus is by

way of a tendon that travels through the trioseal canal and acts as a

pulley to lift the wing. With this design the large supracoracoideus

can remain in the ventral (below the wing) center of gravity while

still being able to raise the wing.

Muscles are composed of both red and white fibers. Red fibers

provide a continuous supply of aerobic power for sustained flight,

fueled by the metabolism of fat and sugar. White fibers are pow-

ered by products of anaerobic respiration and are capable of pro-

viding only short burst of energy for powerful, rapid flight.

Circulatory SystemSustained flight is metabolically demanding and requires an effi-

cient circulatory system to provide oxygen and fuel to the body

while removing toxic metabolites for excretion. Like mammals,

birds have a double circulatory system with a separation of the pul-

monary (to lungs) and systemic (to body) systems to prevent mix-

ing of oxygenated and deoxygenated blood. In birds and reptiles

however, blood passes to the lungs via the left systemic arch,

whereas in mammals blood travels to the lungs via the right sys-

temic arch. Like mammals, birds have a four-chambered heart,

compared to the ancestral reptilian three-chambered heart. The

four-chambered heart is probably necessary to meet the greater

aerobic demands of endothermy. Bird hearts are proportionately

up to 40% larger than mammals while they beat only half as fast as

a similarly sized mammal. The larger avian heart therefore pro-

vides a greater stroke volume of blood to achieve a similar cardiac

output.

Respiratory System The avian respiratory system is extremely efficient in order to meet

the enormous energetic demands of sustained flight. Truly unique

structures and air flow pathway help to distinguish the avian res-

piratory system from that of reptiles and mammals. The air-flow of

reptiles and mammals is two-way, as air passes in and out of the

lungs along the same pathway. Birds, in contrast, have an efficient

one-way air flow system that allows a continuous flow of air to pass

through the lungs. A complete respiratory cycle takes two sets of

inhalation and exhalation (Figure 8). Air sacs (Figure 9) are auxil-

iary structures that help to deliver air through the respiratory tract

but are not directly involved in O2 and CO2 exchange. Oxygen and

CO2 exchange only occurs in the lungs. Air sacs are fairly large and

take up about 15% of the volume of the thorax and the abdomen.

Air sacs also help to dissipate heat, as air passing through the air

sacs absorbs heat and is then expelled from the body. Birds also

lack a diaphragm thus breathing is thus powered by the intercostal

muscles. Finally, instead of using a larynx to produce sound (as in

most mammals), birds use an analogous syrinx (located in the tra-

chea) for sound production.

Digestive SystemThe avian digestive system follows the general vertebrate plan of

mouth, esophagus, stomach, small intestine and large intestine

Musculature

I n t e r na l A nato my 3

Figure 6. Bones of the leg.

PygostylCaudalvertebrae Ilium

Ischium

Pubis

Femur

Fibula

Tibiotarsus

Tarsometatarsus

Lateral condyle

Basal phalanx

Fourth digit Third digit

SeconddigitFirst digit

Calcaneal ridge

Patella

Cnemialcrest

Obturatur foramen

Trochanter of femurIschiadic foremen

Figure 7. Arrangement of the flight muscles (a) cross section through

the sternum (b) lateral view.

Sternum

Trioseal canal

Furcula

CoracoidSupracoracoideus

Scapula

Humerus

Humerus

Scapula

Tendon

SupracoracoideusCoracoid

PectoralisSternum

a

b

(Figure 10). Of course there are many modifications from this gen-

eral plan, due in part to diet and the lack of true teeth and chewing.

In many birds a part of the esophagus is expanded into a crop. The

crop stores and softens food and regulates its flow through the

digestive tract. The stomach of birds is divided into two chambers,

the proventriculus (glandular stomach) and the gizzard (muscular

stomach). Food first flows through the proventriculus where it

receives digestive enzymes. Food then enters the gizzard, which

essentially functions as teeth, where it is mechanically broken

down to maximize surface area contact for the digestive enzymes.

The stomach and crop can vary considerably between species with

different diets. Granivores, for example have a well-developed crop,

proventriculus and gizzard. Birds that primarily consume easily

assimilated fruits, berries, and nectar have a very reduced gizzard.

In many birds, particularly herbivores, the small intestine ends

with blind pouches called caeca. Caeca contain bacteria that aid in

the digestion of cellulose plant material. At the end of the digestive

tract is the cloaca, a holding area for wastes and products from the

digestive and urogenital systems. The cloaca opens to the outside

by means of the vent.

Digestive System

e e e 3 3 54

Inhalation

Expiration

Expiration

Inhalation

Anteriorair sacs

Posteriorair sacs

Lungs

Inhaled air moves down trachea and bronchi to

Air moves from posterior air sacs to the

Air moves from the lungs to the posterior

Air moves out of the body via the trachea.

Figure 8. Schematic diagram of the pattern of air flow through theavian lung.

I n t e r na l A nato my 5

Trachea

Interclavicularair sac

Syrinx

Bronchus

Left lung

Posterior thoracicair sac

Abdominal air sac

Anterior thoracicair sac

Diverticulum to humerus

Cervical air sac

Humeral air sac

Figure 9. The distribution of air sacs and the functioning of the avian lung.

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Esophagus

Crop

Proventriculus

Spleen

Gizzard

Pancreas

Pancreatic ducts

Duodenum

Caeca

Large intestine

Cloaca

Vent

Ureter

Illeum

Mesentery

Jejunum

Bile ducts

Liver

Trachea

Syrinx

Heart

Liver

Gizzard

Duodenum

Lung

Figure 10. Thoracic and abdominal organs.